Add VictoriaMetrics / Grafana Mimir support to the Prometheus backend

[mayankpande88]

VictoriaMetrics and Grafana Mimir both speak the Prometheus query API, but with small differences (URL base path and multi-tenancy headers such as AccountID/X-Scope-OrgID). Today the agent's Prometheus backend assumes vanilla Prometheus, so users running VM or Mimir can't point the agent at them cleanly.

Goal

Make the existing Prometheus observability backend work against VictoriaMetrics and Mimir, either via config knobs on the existing client or a thin backend variant.

Scope

• Support a configurable base path / URL prefix (e.g. VM's /select/0/prometheus, Mimir's /prometheus).
• Support an optional tenant header (X-Scope-OrgID for Mimir, AccountID/tenant for VM).
• Surface these as values.yaml config on the runner.
• Unit test covering the path + header difference using httptest.Server.
• Add a values.yaml example and a short docs note.

Pointers

• runner/pkg/observability/prometheus/
• runner/docs/development.md → "Adding a new action"
• runner/docs/configuration.md for the env-var surface

Acceptance

• make test passes with new coverage for the tenant-header / base-path handling.
• A documented config example lets a user target VM or Mimir without code changes.